1. Field of the Invention
The present invention relates to die layouts for electronic circuits, and, more specifically, to die layouts for surface-acoustic-wave devices.
2. Description of Related Art
SAW devices typically comprise interdigitated conductive electrode patterns (transducers) and conductive electrode grating patterns on a surface of piezoelectric materials. When an alternating-polarity electrical signal is applied to the transducer of these devices, a surface acoustic wave is launched at the surface of the piezoelectric material. The electrode grating regions of the device serve to reflect the surface wave due to mechanical and electrical effects. The interdigitated conductive electrode patterns of the transducer have electrode regions and bus-bar regions. The transducer electrode regions serve to stress the piezoelectric and generate an acoustic wave when signals of alternating polarity are applied, while the bus-bar regions serve to electrically connect the individual electrodes in the electrode region to one polarity or the other, and to transmit the applied voltage to the electrodes. The conductive bus-bar regions that are connected to device electrical terminals to which voltage is applied are usually called bond pads, while the conductive bus-bar regions that are electrically connected to ground are generally called ground pads.
The gratings also have electrode regions and bus-bar regions. The electrode regions serve to reflect the surface acoustic wave, while the bus-bar regions serve to electrically connect the individual electrodes in the electrode region to one polarity.
SAW device types known as coupled resonator filters (CRFs) utilize multiple resonant regions coupled acoustically and electrically in configurations designed to achieve a desired frequency response. A typical CRF structure comprises two parallel tracks, each consisting of a central transducer with two bus bars and bond pads, two additional interdigitated transducer regions on either side of the central transducer, and two reflective gratings at the outer ends of this transducer structure. The xe2x80x9chotxe2x80x9d bond pads for the transducers are isolated. The outer transducers do not have bond pads, but rather one bus bar polarity is connected with the ground pads of the adjacent gratings, and the other polarity bus bar is connected electrically to the bus bar of the similar transducer in the second track. Thus these transducers are electrically connected, and the nongrounded sides of these transducers are xe2x80x9cfloatingxe2x80x9d relative to the electric potential of the central transducers. Within each track, a resonance is established when the central transducer launches an acoustic wave. The series-connected floating transducers serve to couple the signal between the two tracks. A typical CRF layout 10 with isolated central transducers 11, 12 is shown by way of example in FIG. 1. It should be noted that all the figures presented herein have been simplified and enlarged to show the details of the device layout. Typical devices would have many more electrodes than illustrated.
Traditionally, for CRF devices the electrode regions have been oriented perpendicular to the direction of surface-wave propagation and parallel to the xe2x80x9cendsxe2x80x9d 13, 14 of the SAW device die 10. In this traditional configuration, the bus bars are generally parallel to the other set of die edges (the xe2x80x9csidesxe2x80x9d 15, 16 of the die). This type of layout 10 is shown in FIG. 1. Electrical performance requirements make it essential for the isolated xe2x80x9chotxe2x80x9d bond pads 17, 18 to be placed in close proximity to, and in good electrical contact with, the central transducer bus bar 19, 20. Manufacturing requirements dictate minimum sizes for bond pads, and that certain blank regions of crystal be left between the edge of the die and any metallized region, be it electrode or bus bar. This is particularly important for devices that are mounted using flip-chip technology, where the bumps formed on the bond pads can be torn off at the time the dies are singulated if the bond pads are not set far enough back from the die edge.
It can be seen from FIG. 1 that the bus-bar and bond pad sizes and bare die regions add to the overall size of the die 10. The choice of package size is dependent on how small the die can be made for a given level of electrical performance. Customers typically have requirements for electrical performance and desire as small a package size as possible. Thus improvements in die layout that result in a reduction in die size for a given electrical performance allow for an overall reduction in package and device size. In this case, reduction in the length of the die will result in package size reduction.
Historically, the die edges nominally perpendicular to the direction of surface-wave propagation have been called the xe2x80x9cendsxe2x80x9d 13, 14 of the die 10, and the spatial extent of the die perpendicular to these ends 13, 14 has been called the die width 21. The die edges parallel to the direction of surface-wave propagation have been called the xe2x80x9csidesxe2x80x9d 15, 16 of the die. We will retain this terminology of die ends, sides, length, and width, even though the CRF devices under consideration often have aspect ratios such that the width may exceed the length. Generally CRF devices are small enough in the length dimension to fit into the small packages desired by the customer, but the problem arises in the width of the die required. In order to fit multiple transducer apertures and multiple (often 3 or more) bus bars and bond pads across the width of a die, the die width must be made substantially larger than necessary for the active electrode region alone. Specifically, the need to maintain the isolated hot bond pads 17, 18 for the two central transducers 11, 12 in close proximity to, and in good electrical contact with, the central transducers"" bus bars 19, 20 dictates that the largest dimension on the die electrode layout be the length 22 from the outer edge of one central transducer""s 11 hot bond pad 17 to the outer edge of the other central transducer""s 12 hot bond pad 18. This dimension can be reduced as much as possible within manufacturing tolerances, but even when reduced as much as possible (while maintaining electrical performance and impedance characteristics), this dimension is the limiting factor in reducing die size further.
Further features of the prior art die 10 include four gratings 23, two surrounding each central transducer 11,12, and four bond pads 24 leading thereto. Generally square bond pads 17,18,24 are in this design generally collinear and are adjacent the sides 15,16 of the die 10, with their outer edges generally parallel the sides 15,16, and their top and bottom edges generally parallel the ends 13,14. In such an embodiment the width 21 is smaller than the length 22 of the die 10.
Whereas in FIG. 1, the two bond pads 25, 26 between the transducer/grating array are separate and in spaced relation to each other, FIG. 2 illustrates another prior art embodiment 10xe2x80x2 in which the two bond pads 25xe2x80x2, 26xe2x80x2 have been joined, reducing the die width 21xe2x80x2 and length 22xe2x80x2 slightly.
It is therefore an object of the present invention to provide a die layout that reduces die width.
It is another object to provide such a die layout that increases efficiency.
It is a further object to provide such a die layout that reduces package size.
It is an additional object to provide such a die layout that results in a reduced device size.
These and other objects are achieved by the present invention, a surface-acoustic-wave device. The device comprises a generally rectangular die that comprises a piezoelectric material. A surface-acoustic-wave electrode pattern is positioned atop the die. The pattern has a generally rectangular footprint, and the footprint has a top edge that is positioned at an acute, nonzero angle to a top end of the die. The die further comprises a pair of generally rectangular electrode pads. Both of the electrode pads are in electrical contact with the electrode assembly, such that a first pad is adjacent a first corner formed by a bottom end and a first side of the base, and a second pad is adjacent a second corner formed by the top end and a second side of the base, which is opposed to the first side.
The present invention addresses the inefficiency in the conventional die layout technique, and provides a general method for reducing the width of the die (reducing the spatial extent of the longest die edge) for SAW CRF filters. This reduction in die width yields a possibility of a smaller package, resulting in an overall smaller device, which can be more efficient and more competitive in the marketplace.
The features that characterize the invention, both as to organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description used in conjunction with the accompanying drawing. It is to be expressly understood that the drawing is for the purpose of illustration and description and is not intended as a definition of the limits of the invention. These and other objects attained, and advantages offered, by the present invention will become more fully apparent as the description that now follows is read in conjunction with the accompanying drawing.